Things can go wrong after payment. Lying about receiving a card, claiming it's damaged when it wasn't, being too unreasonable like badgering a seller for updates, etc. The actual payment is not the only part of the transaction to me as a buyer. Feedback by both sides should be done when the transaction is totally complete which happens when the buyer has the card in hand.

Again... how are you determining that? When DC says it's delivered? We've all seen plenty of cases where DC says delivered when it's not, and even when it is, the buyer claiming never to have received it.